In the power system of a motorcycle, generally an engine is mounted on a body. A rear fork formed of pipe members is pivoted at one end to the intermediate rear part of the body. A rear wheel is borne at the other end, and is connected at said other end with the body side through cushion units to be swingable including the rear wheel. The rear wheel and engine are connected with each other through a chain or the like.
In the above power system, because the engine and the rear fork supporting the rear wheel are separated from each other, and the rear wheel including the rear fork swings, a chain tensioner for the chain (or a belt tensioner in case a belt is used) will be required and the mechanism including its supporting means will be complicated. Therefore, such system is adopted in a somewhat large motorcycle. In case a small light motorcycle is to be simplified, a simplified system in which the power system moves integrally is desirable.
Where the power system moves integrally, the unsprung weight for the cushion necessarily becomes so large that the life and durability of the shock absorbers, the function, and the riding comfort are reduced.
Therefore it is necessary to lighten and simplify the power system including the rear wheel suspending supporting means, and a sufficient strength must be retained while attaining the above.
In case such system mentioned above is adopted, as the engine moves, it will be very difficult to arrange a suction system in a proper position. Particularly, it is not desirable to provide an air cleaner and suction pipe on the body side because the entire power unit swings, but it is desirable to provide them within this unit to swing integrally with the engine.
In case the above system is adopted, the position of housing and mounting an ignition coil will be a problem. Generally, the ignition coil is housed below a seat or a tank. In such case, the high tension cord will be so long as to be undesirable for wiring and appearance. However, in case the problem of heat is considered, it will not be desirable to place the coil near the engine.
In case the above system is adopted, a chain or belt which is a power transmitting member will be arranged on the side of the unit. However, when it is exposed, noise will be produced when driving and rainwater or muddy water will be applied to it. containing the chain or belt in a semi-sealed box-shaped body can effectively protect the chain from rainwater or muddy water, but is a poor countermeasure for preventing noises. Vibrations of the drive of the chain or belt will resonate with the sealed box-shaped body to cause such closed sounds as growls, and the closed sounds will emanate out through the clearances of the box-shaped body to generate noises.